1. Field of the Invention
The present invention relates to the preparation of fuel components. In particular, the invention concerns a process for dimerizing and etherifying isoolefins in a reactor train system comprising, in a cascade, a primary reaction zone, a primary distillation zone, a secondary reaction zone and a secondary distillation zone.
2. Description of Related Art
Tertiary alkyl ethers are added to automotive fuels (gasoline) in order to improve the anti-knocking characteristics of the fuels without using organolead compounds and in order to reduce the concentration of detrimental components in the exhaust gases. The oxygen-containing ether group of these octane-booster compounds has been found to improve the combustion process favourably. Suitable alkyl tert-alkyl ethers are methyl t-butyl ether (MTBE), ethyl t-butyl ether (ETBE), t-amyl methyl ether (TAME), t-amyl ethyl ether (TAEE) and t-hexyl methyl ether (THME), just to mention a few examples. These ethers are prepared by etherification of a monovalent aliphatic alcohol with an isoolefin. These olefins include, but are not limited to isobutene, 2-methyl-1-butene (2M1B), 2-methyl-2-butene (2M2B), 2-methyl-1-pentene (2M1P), 2-methyl-2-pentene (2M2P) and 2,3-dimethyl-1-pentene (2,3DMP).
Processes for producing the above-mentioned ethers are disclosed in, e.g., U.S. Pat. Nos. 5,536,886, 5,637,777, 5,908,964, and 6,369,280.
The octane number of the automotive fuels can also be increased by adding other gasoline components, such as C4-alkylates or isomerates. The alkylate is typically produced by alkylating isobutane and isobutene, whereby trimethylpentanes and dimethylhexanes are obtained. Furthermore, by dimerizing isobutene to isooctene, and optionally further hydrogenating it to isooctane, a component equal to or better than alkylate is obtainable. Isooctane/isooctene processes are discussed in, e.g., EP Patent Application No. 0 994 088, U.S. Pat. No. 6,011,191 and GB Patent No. 2 325 237.
Various processes of producing both olefin ethers and olefin dimers in the same reaction system are also known in the art. The reaction between an isoolefin and an alcohol is an equilibrium reaction which, depending on the reaction conditions, will yield an ether or a dimer, or (a mixture of) both. Thus, to mention an example, EP-A-0 745 576 discloses a process, in which MTBE and isooctene are produced simultaneously. According to the publication, the molar ratio of the alcohol and the isoolefin has to be primarily sub-stoichiometric or in the range of 0.2-0.7.
Another process for producing both C4-oligomers and alkyl-t-butylether is known from EP-A-0 048 893. In that process, a high feed ratio of alcohol and isobutene is used. In the publication, reference is made to the possibility of recycling the product in order to produce longer oligomers.
EP-A-0 994 088 concerns an improved process for dimerizing iso-olefins, which employs a reactor train system including a reaction zone and a separation zone (e.g. a distillation column) connected to an effluent outlet of the reaction zone. The olefinic hydrocarbon feedstock is contacted with an acidic catalyst in the presence of an oxygenate at conditions in which at least a part of the olefins dimerizes, the effluent is conducted from the reaction zone to the separation zone, where dimerized reaction product is separated, and the reaction product is then recovered and, optionally hydrogenated. A side draw comprising unreacted olefins and alkanol is taken from the separation zone and recirculated to the reaction zone.
The process described in EP-A-0 994 088 is particularly advantageous because it allows for free selection of the product composition of the dimerizing unit and makes it possible to produce either pure dimer or a mixture of dimer and ether in the same unit.
Although the consumption of alkyl ethers as octane-boosters is decreasing in certain areas of the world, such as in California, many producers still wish to retain the option of using the same basic equipment for producing a dimer, such as isooctane/isooctene, and alkyl ethers, such as methyl-t-butyl ether or t-amylmethyl ether, depending on the demand on the market for the various fuel components. In particular, there is a need for ways of easy modification of the equipment used for dimerization of olefins so that it can be used for etherification of the olefins at high conversion rates.